The present invention relates to a humidifier for use with a fuel cell, and more particularly to a humidifier utilizing water permeable membranes.
Fuel cell systems, especially solid polymer fuel cell systems are widely known as a power source for electric vehicles. In such fuel cell systems, a humidifier is used to moisture-exchange off gas, viz. moist gas discharged from a fuel cell, between its moisture and air as a dry gas, and to generate humidified air or humidified gas. Preferably, a humidifier used with such fuel cell systems is of a lower power consumption type, and is required compactness with less attachment space. For this reason, among many other humidifiers, such as a supersonic humidifier, a steam humidifier, a vaporizing humidifier and a nozzle injection type humidifier, a humidifier utilizing water permeable membranes, especially hollow fiber membranes is commonly used with a fuel cell.
A conventional humidifier utilizing hollow fiber membranes is disclosed in Japanese Laid-open Patent Publication No. HEI-7-71795. As shown in FIG. 12, a humidifier 100 comprises a housing 101, on which is provided a first inlet 102 for introducing dry air and a first outlet 103 for discharging the dry air (humidified dry air). A bundle of hollow fiber membranes 104 consisting of a number of for example 5000 hollow fiber membranes is accommodated within the housing 101.
At both ends of the housing 101, fastening members 105, 105xe2x80x2 are provided for fixing the ends of the bundle 104 while leaving them open. Outside of the fastening member 105 is provided a second inlet 106 for introducing moist air or moist gas, and a second outlet 107 is provided outside of the fastening member 105xe2x80x2 for discharging the moist air, moisture of which is separated and removed at the bundle of hollow fiber membranes 104. The fastening members 105, 105xe2x80x2 are covered with a first head cover 108 and a second head cover 109, respectively. And the second inlet 106 is formed on the first head cover 108, while the second outlet 107 is formed on the second head cover 109.
In the aforementioned humidifier 100 utilizing hollow fiber membranes, the moist air introduced from the second inlet 106 passes through the hollow fiber membranes forming the bundle of hollow fiber membranes 104, and the moisture within the moist air is separated by capillary action of the hollow fiber membranes. The separated moisture moves outward of the hollow fiber membrane through a capillary tube of the membrane. The moisture-removed air is discharged from the second outlet 107.
Meanwhile, dry air is supplied from the first inlet 102. The dry air from the first inlet 102 flows outside of the hollow fiber membranes forming the bundle of hollow fiber membranes. Because the moisture separated from the moist air has moved outside of the hollow fiber membranes, the moisture humidifies the dry air. The humidified dry air is then discharged from the first outlet 103.
However, as shown in FIG. 13, the conventional humidifier 100 has humidifying characteristics, on the basis of output of the fuel cell, in which the dew point of the humidified air decreases both in a low power output range where small amount of humidified air is required and a high power output range where large amount of humidified air is required, and the dew point of the humidified air increases in a middle power output range where middle amount of humidified air is required. Because dried electrolyte membranes affect fuel cells, the humidifying process is generally carried out on the basis of the low power output range or the high power output range. Therefore, the fuel cell tends to be excessively humidified in the middle power output range. As a result of the excessive humidification, if diffusion layers or electrodes of the fuel cell are immersed in water, free flow of the gas is interrupted in the fuel cell, which leads to a drawback in that expected performance is not achieved.
In order to improve the humidifying capability of the humidifier, a large number of hollow fiber membranes are required to provide extensive surface area of the membranes. However, with increasing the number of hollow fiber membranes, dry air flowing outside of the membranes tends to arise a drift, and the dry air does not equally flow throughout the housing. Thereby, the extensive surface area of the outer surface of the hollow fiber membranes cannot be used effectively, which leads to decreased humidifying efficiency and hence to a drawback that it is impossible to increase the dew point. This can also be said to other water permeable membranes.
In view of the above, the present invention seeks to provide a humidifier for use with a fuel cell, which can supply a fuel cell with humidified gas with stable dew points at the entire range from a low power output range to a high power output range, and which can utilize the extensive outer surface area of the hollow fiber membranes.
As the result of intensive researches, the inventors eventually complete the invention by focusing the reason why the dew point of the humidified air through the humidifier has the characteristics such as shown in FIG. 13. Namely, in a low power output range of the fuel cell, the dew point of the humidified air cannot rise sufficiently, because the required amount of the humidified air is small and the operating pressure of the fuel cell is low. Meanwhile, in a high power output range of the fuel cell, the dew point of the humidified air cannot rise sufficiently, because the required amount of the humidified air is large.
According to the present invention, the objects of the invention can be accomplished by providing a humidifier for use with a fuel cell, comprising:
a plurality of combined water permeable membranes or water permeable devices, each of the water permeable membrane or water permeable device generating humidified gas by flowing therein different gases with different moisture contents and by moisture exchanging between the different gases so that one dry gas with smaller moisture content is humidified with the other moist gas with larger moisture content; and
flowing passage switching means for optionally switching flowing passages of said dry gas;
wherein said flowing passage switching means switches the flowing passage in accordance with a required amount of said humidified gas so as to selectively use particular water permeable membranes or a particular water permeable device from among the plurality of water permeable membranes or water permeable devices.
In such a construction, even if the required amount of the humidified gas is increased (output of the fuel cell is increased), control can be made with the flowing passage switching means so that the dew point of the humidified gas is converged within a certain extent. As a result, a humidifier suitable for use with a fuel cell can be provided. In this instance, in view of leveling the dew point, regardless of the required amount of the humidified gas, it is preferable to control the water permeable membranes or water permeable devices so as to be operable within a center region of the graph (FIG. 13), where the dew point is stable. The xe2x80x9coff gasxe2x80x9d appeared in the preferred embodiment corresponds to the xe2x80x9cmoist gasxe2x80x9d, and the xe2x80x9cdry airxe2x80x9d and xe2x80x9chumidified airxe2x80x9d correspond to the xe2x80x9cdry gasxe2x80x9d and xe2x80x9chumidified gasxe2x80x9d, respectively. Further, the xe2x80x9chollow fiber membranesxe2x80x9d or xe2x80x9cbundle of hollow fiber membranesxe2x80x9d in the preferred embodiment corresponds to the xe2x80x9cwater permeable membranesxe2x80x9d, and the xe2x80x9chollow fiber membrane modulexe2x80x9d corresponds to the xe2x80x9cwater permeable devicexe2x80x9d. The water permeable membranes may be in the form of a repeatedly folded film-like water permeable membrane or in the form of a winded roll.
According to a preferred embodiment of the present invention, each of the plurality of combined water permeable membranes or water permeable devices has a different humidifying capability.
In such a construction, the humidified gas is generated in accordance with the required amount of the humidified gas (the output of the fuel cell) so that when the required amount of the humidified gas is smaller, water permeable membranes or a water permeable device with a relatively low humidifying capability is used, and when the required amount is larger, water permeable membranes or a water permeable device with a relatively high humidifying capability is used. As a result, the dew point can be properly converged within a certain extent regardless of the required amount of the humidified gas. In the case that the water permeable membranes are hollow fiber membranes, the humidifying capability thereof can be varied by changing its profile, inner diameter, shape, length, or the material of the hollow fiber membrane. Also, the humidifying capability can be varied by changing the number of hollow fiber membranes retained in the hollow fiber membrane module or the bundle of hollow fiber membranes.
As a plurality of combined water permeable membranes or water permeable devices, the following three cases are available, in which:
(1) A plurality of water permeable devices are employed, and with the use of the flowing passage switching means, a suitable water permeable device is selected or alternatively the number of water permeable devices to be used is increased or decreased, in accordance with the required amount of the humidified gas;
(2) Only one water permeable device is employed. However, the water permeable device consists of a plurality of water permeable membranes, and with the use of the flowing passage switching means, suitable water permeable membranes are selected or alternatively the number of water permeable membranes to be used is increased or decreased, in accordance with the required amount of the humidified gas. In this instance, the moist gas and the dry gas (humidified gas) are separated by the water permeable membrane. However, a separating wall is preferably provided within the water permeable device so as to separate a gas to be flown though one water permeable membrane assembly from the other gas to be flown through the other water permeable membrane assembly; and
(3) Combination use of (1) and (2).